Short Answer: Backlinks are all links coming into a website from internal and external websites. These backlinks are used to help boost page ranking, relevancy and authority.
What are links and how are they used?
Firstly what is a link? A link is any part of a website that you can click on which leads to another page on the same site or to another site. Backlinks describe the incoming links on a particular web page. Take this diagram below as an example: A number of web pages are directly linking to a particular website building a strong navigational relationship.
How do backlinks help my site?
Backlinks can play a role in determining your ranking position on search engines and obtaining links in quantity and quality can heavily influence your search engine rankings. Essentially, a backlink acts as a positive ‘vote’ for that page. Theoretically, the more votes the better the chance that page has of ranking higher. However, links from relevant industry sources are more likely to improve rankings than links from an unrelated industry source.
In summary, backlinks are an important ingredient to the overall structure of websites, providing navigational guidance as well as influencing the authority, relevancy and ranking of a page.
Image by Search Influence Flickr